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Re: Cantilevered Wood Diaphragm

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In Los Angeles and other cities in So. Cal, we would not be able to
design this with cantilevers as you have attempted to do. We would have
to use moment frames on the exterior walls.

Stan Scholl, P. E.
Laguna Beach, CA

On Sun, 15 Oct 2000 13:56:39 -0800 Monty Hart <montyh(--nospam--at)gci.net> writes:
> I recently designed a wood framed, two story, apartment building for
> lateral loads.  In the longitudinal direction, there are no exterior
> shear walls, however, there are longitudinal shear walls along a 6' 
> wide
> corridor along the center of the building.  The floor and roof
> diaphragms are cantilevered out about 25' on each side of the 
> corridor
> walls (building is about 56' wide x 184' long).  There are 
> transverse
> shear walls at about 46' oc between living units.  The shear wall 
> plan
> looks something like this;
> 
>    46'      46'      46'     46'
> |          |          |          |          |  25'
> |          |          |          |          |
> ------------------------------------
> ____________________    6'
> |          |          |          |          |
> |          |          |          |          |  25'
> 
> Shear walls on the first and second floors stack, seismic governs 
> (Zone
> 4), diaphragms are assumed flexible.  I am analyzing the diaphragm 
> as a
> continuous 184' deep horizontal beam symmetrically cantilevered out 
> 25'
> on each side of the building.  I maintain that because the seismic
> loading and the ("H shaped") geometry are symmetrical, there is no
> rotation that would distribute forces to the transverse shearwalls.
> 
> The municipal plan checker, who is a licensed engineer, has stated 
> that
> the shear walls must be designed for diaphragm rotation, with each 
> of
> the living units (bounded by it's three shear walls) acting as a 
> "three
> sided box".  The plan checker's written comment on this issue is as
> follows;
> 
>            "Engineers assertion that the diaphragm will not rotate
> because the center of mass is at the
>            geometric center is not correct.  Flexible diaphragms
> distribute forces to shear walls based on location and
>            not the rigidity of the elements.  Only rigid diaphragms 
> act
> in that way.  Also, the shear walls break the
>            diaphragms into smaller pieces, and these pieces are not 
> H
> shaped as assumed.  Each small diaphragm
>            acts in rotation, although the forces on the center 
> elements
> may be canceling each other out- that is
>            subject to analysis.   Please check all three-sided 
> elements,
> and provide design for the diaphragm, the chord forces, the nail
>            slip, and the added shear and overturning of the back 
> walls
> as a result of rotation forces.  Show all
>            elements on the plans."
> 
> If this was a simple "three sided box" structure, I would certainly
> agree with the plan checker about designing for rotation.  However, 
> I do
> not believe the condition is comparable.  I have been unable to 
> locate
> any literature that addresses the design of a symmetrically 
> cantilevered
> flexible diaphragm, although I know it is not that unusual in 
> apartment
> buildings.  Any opinions or information, pro or con, would be 
> greatly
> appreciated.
> 
> Monty Hart
> Associated Design Consultants, Inc.
> Anchorage, Alaska
> www.ADC-Engineers.com
> 
> 
> 
> 

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